9. Johnny Depp - 21 Jump Street
21 Jump Street was a reboot of an eighties TV show that starred a then-rising Johnny Depp. And although the references were ultimately slight (let's save the meta humour for the sequel) Phil Lord and Christopher Miller were clearly aware of the movie's origins. Naturally this sparked questions over whether the A-list star would honour his roots and have a little cameo. Although little fuss was made about it, that's exactly what happened.
The reboot's final act revealed that one of the biker drug dealers was in fact an undercover cop, played by none other than Depp. To those new to Jump Street it's great simply because it's Johnny Depp, but for those old enough to remember the show it's further proof that Lord and Miller were totally the right guys for the job. What makes Depp's appearance so delightful is that it's more than just a wink to fans; it places Schmidt and Jenko in the same world as Hanson.
Depp had some conditions for doing the cameo, and not the usual contractual issues that you'd expect; he wanted original co-star Peter DeLuise to also appear and for Hanson to die.